The land mobile radio service has been in existence for some decades. With few exceptions, the service has been restricted to voice communication via frequency-modulation channels. Current practice is to use a central base-station in a given service area, transmitting sufficient power to cover the entire area with adequate field strength. In some cases, satellite stations transmitting the same message may be used to boost signal strength in hard-to-reach areas (the practice known as "simulcasting").
Much recent technical literature has been addressed to the use of a cellular structure as a means of increasing the efficiency of the land-mobile radio service. The proposal is to divide the service area into cells (most often hexagons), each with a base station at its center. The communication link is completed by land line, with radio channels bridging the small gap between each mobile and its nearest base station. If a mobile moves to a new cell, the radio link is switched to the new base station.
Hence, while mobile radio service is now provided, improvements therein have been needed to enable greater satisfactory use of such service, particularly in providing adequate area coverage and in providing more efficient use of the available frequency spectrum. In addition, while improvements such as cellular communications within a service area have heretofore been proposed, such proposals are not fully satisfactory in solving many of the problems presented and/or create new problems.
One such suggested approach has been to provide a unique set of time-frequency coded waveforms to each user for both transmission and reception. The use of such an approach requires the use of a demodulation unit capable and suitable for accomplishing the desired end.